No. 522] 



PHYSIOLOGICAL L'KSl'OXSL 



327 



edge of the energetics of plants does not enable us to de- 

 cide. Certainly we can not accept the source proposed 

 by Bose, when he writes : 



It is of course possible to use the term stimulus to 

 designate any external or internal agent that produces an 

 effect upon the organism, and response for the effect 

 produced; but that use of the terms tends to confusion. 

 Yet, besides the erratic treatises of Bose, some recent 

 text-books ii^e the words in this way. One, for example, 

 reads: "Responses to water stimuli.— The primary re- 

 sponses of the plant to the water of the habitat are four: 

 namely, absorption, diffusion, transport, and transpira- 

 tion." It is hardly necessary to point out that if such 

 processes are responses, then every change that occurs 

 in nature is a response and every agent a stimulus. By 

 such a usage we should lose the whole value of the dis- 

 crimination embodied in the term-. 



The less clearly plant processes are conditioned by the 

 environment, the more likely they are to be reckoned 

 responses to stimuli. Growth, for example, is often con- 

 sidered a response to stimuli; but it seems most likely 

 that it is quantitatively determined by various factors 

 (turgor, temperature, oxygen, food, etc.), any one of 

 which may limit it. Stimulation by gravity or light may 

 be an added factor, interlocking with one or more of 

 them, and, by inducing local variation in the rate of 

 growth, producing curvature; which is obviously a 

 physiological response, for when the effect of the stimu- 



In considering the energetics of response, it is essen- 

 tial not to forget that many stimuli inhibit actions going 

 on at the time excitation occurs. There are many famil- 

 iar examples of this among animals, but few in plants. 



